1. Field of the Invention
The invention relates to gelled fracturing fluids used in well bore operations. More specifically, the present invention relates to methods of hydrolyzing gelled fracturing fluids using enzymes incorporated in the gelled fracturing fluids, particularly in environments having elevated pH values.
2. Description of the Related Art
Hydraulic fracturing is used to create subterranean fractures that extend from the borehole into rock formation in order to increase the rate at which fluids can be produced by the formation. Generally, a high viscosity fracturing fluid is pumped into the well at sufficient pressure to fracture the subterranean formation. In order to maintain the increased exposure to the formation, a solid proppant is added to the fracturing fluid which is carried into the fracture by the high pressure applied to the fluid.
Some conventional fracturing fluids include guar gum (galactomannans) or guar gum derivatives, such as hydroxypropyl guar (HPG), carboxymethyl guar (CMG), or carboxymethylhydroxypropyl guar (CMHPG). These polymers can be crosslinked together in order to increase their viscosities and increase their capabilities of proppant transport.
Once the formation is adequately fractured and the proppant is in place, the fracturing fluid is recovered typically through the use of breakers. Breakers generally reduce the fluid's viscosity to a low enough value that allows the proppant to settle into the fracture and thereby increase the exposure of the formation to the well. Breakers work by reducing the molecular weight of the polymers, which “breaks” the polymer. The fracture then becomes a high permeability conduit for fluids and gas to be produced back to the well.
Besides providing a breaking mechanism for the gelled fluid to facilitate recovery of the fluid, breakers can also be used to control the timing of the breaking of the fracturing fluids, which is important. Gels that break prematurely can cause suspended proppant material to settle out of the gel before being introduced a sufficient distance into the produced fracture. Premature breaking can also result in a premature reduction in the fluid viscosity resulting in a less than desirable fracture length in the fracture being created.
On the other hand, gelled fluids that break too slowly can cause slow recovery of the fracturing fluid and a delay in resuming the production of formation fluids. Additional problems can result, such as the tendency of proppant to become dislodged from the fracture, resulting in a less than desirable closing and decreased efficiency of the fracturing operation.
For purposes of the present application, premature breaking will be understood to mean that the gel viscosity becomes diminished to an undesirable extent before all of the fluid is introduced into the formation to be fractured.
Optimally, the fracturing gel will begin to break when the pumping operations are concluded. For practical purposes, the gel should be completely broken within a specific period of time after completion of the fracturing period. At higher temperatures, for example, about 24 hours is sufficient. A completely broken gel will be taken to mean one that can be flushed from the formation by the flowing formation fluids or that can be recovered by a swabbing operation. In the laboratory setting, a completely broken, non-crosslinked gel is one whose viscosity is either about 10 centipoises or less as measured on a Model 50 Fann viscometer Rl/Bl at 300 rpm or less than 100 centipoises by Brookfield viscometer spindle #1 at 0.3 rpm.
By way of comparison, certain gels, such as those based upon guar polymers, undergo a natural break without the intervention of chemical additives. The break time can be excessively long, however. Accordingly, to decrease the break time of gels used in fracturing, chemical agents are incorporated into the gel and become a part of the gel itself. Typically, these agents are either oxidants or enzymes that operate to degrade the polymeric gel structure.
However, obtaining controlled breaks using various chemical agents, such as oxidants or enzymes, has proved difficult. Common oxidants are ineffective at low temperature ranges from ambient temperature to 130° F. The common oxidants require either higher temperatures to cause homolytic cleavage of the peroxide linkage or a coreactant to initiate cleavage. Common oxidants do not break the polysaccharide backbone into monosaccharide units. The breaks are nonspecific, creating a mixture of macromolecules. Further, common oxidants are difficult to control because they not only attack the polymer, but they also react with any other molecule that is prone to oxidation. Oxidants can react, for example, with the tubing and linings used in the oil industry, as well as, resins on resin coated proppants.
Enzymes, on the other hand, are catalytic and substrate specific and will catalyze the hydrolysis of specific bonds on the polymer. Using enzymes for controlled breaks circumvents the oxidant temperature problems, as the enzymes are effective at the lower temperatures. An enzyme will degrade many polymer bonds in the course of its useful lifetime. Unfortunately, enzymes operate under a narrow pH range and their functional states are often inactivated at high pH values. Conventional enzymes used to degrade galactomannans have maximum catalytic activities under mildly acidic to neutral conditions (pH 5 to 7). Activity profiles have indicated that the enzyme retains little to no activity past this point. Enzymatic activity rapidly declines after exceeding pH 8.0 and denatures above pH 9.0. In the case of borate crosslinked guar gels, the gels are also pH dependant requiring pH in excess of 8.0 to initiate gellation. As the pH increases, the resulting gel becomes stronger. Normally, when enzymes are used with borate crosslinked fluids these gels are buffered to maintain a pH range of 8.2 to 8.5 to ensure both gellation and enzymatic degradation. This technique requires high concentrations of both borate and enzyme. Unfortunately, while ensuring good breaks, the initial gel stability and proppant transport capability is weakened. The determination of the optimum enzyme concentration is a compromise between initial gel stability and an adequate break.
Because most guar polymers are crosslinked at pH values between 9.5 and 11.0 for fracturing applications, a need exists for a breaker that can degrade guar-based fracturing fluids within that range, such as at pH ranges ≧10.5. A need also exists for a gel system for a well fracturing operation that can break the gel polymers within a wide range of pH values at low to moderate temperatures without interfering with the crosslinking chemistry. It would be advantageous to provide an enzyme breaker system for a gelled fracturing fluid that produces a controlled break over a wide pH range and at low temperatures and that decreases the amount and size of residue left in the formation after recovery of the fluid from the formation.